The present invention relates to electrostatic inkjet print technologies and, more particularly, to printheads and printers of the type such as described in WO 93/11866 and related patent specifications.
Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location. One particular advantage of this type of print technology over that of conventional drop-on-demand (DOD) printers is the ability to eject continuously variable ink volume, something which is not possible with conventional DOD printers.
The location from which ejection occurs is determined by the printhead geometry and the position and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and ejection occurs at a highly localised region around the tip of the protrusions.
To operate reliably, the ink must flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoise. The material that is ejected is more viscous because of the concentration of particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 01/30576 and WO 03/101741, all of which relate to the so-called Tonejet® method described in WO 93/11866.
FIG. 1 is a drawing of the tip region of an electrostatic printhead 1 of the type described in this prior art, showing several ejection upstands 2 each with a tip 21. Between each two ejection upstands is a wall 3, also called a cheek, which defines the boundary of each ejection cell 5. In each cell, ink flows in the two pathways 4, one on each side of the ejection upstand 2 and in use the ink meniscus is pinned between the top of the cheeks and the top of the ejection upstand. In this geometry the positive direction of the z-axis is defined as pointing from the substrate towards the printhead, the x-axis points along the line of the tips of the ejection upstands and the y-axis is perpendicular to these.
FIG. 2 is a schematic diagram in the x-z plane of a single ejection cell 5 in the same printhead 1, looking along the y-axis taking a slice through the middle of the tips of the upstands 2. This figure shows the cheeks 3, the ejection upstand 2, which defines the position of the ejection location 6, the ink pathways 4, the location of the ejection electrodes 7 and the position of the ink meniscus 8. The solid arrow 9 shows the ejection direction and also points towards the substrate. Each upstand 2 and its associated electrodes and ink pathways effectively forms an ejection channel. Typically, the pitch between the ejection channels is 168 μm (150 channels per inch). In the example shown in FIG. 2 the ink usually flows into the page, away from the reader.
FIG. 3 is a schematic diagram of the same printhead 1 in the y-z plane showing a side-on view of an ejection upstand along the x-axis. This figure shows the ejection upstand 2, the location of the electrode 7 on the upstand and a component known as an intermediate electrode (10). The intermediate electrode 10 is a structure that has electrodes 101, on its inner face (and sometimes over its entire surface), that in use are biased to a different potential from that of the ejection electrodes 7 on the ejection upstands 2. The intermediate electrode 10 may be patterned so that each ejection upstand 2 has an electrode facing it that can be individually addressed, or it can be uniformly metallised such that the whole surface of the intermediate electrode 10 is held at a constant bias. The intermediate electrode 10 acts as an electrostatic shield by screening the ejection channel from external electric fields and allows the electric field at the ejection location 6 to be carefully controlled.
The solid arrow 11 shows the ejection direction and again points in the direction of the substrate. In FIG. 3 the ink usually flows from left to right.
In operation, it is usual to hold the substrate at ground (0 V), and apply a voltage, VIE, between the intermediate electrode 10 and the substrate. A further potential difference of VB is applied between the intermediate electrode 10 and the electrodes 7 on the ejection upstand 2 and the cheeks 3, such that the potential of these electrodes is VIE+VB. The magnitude of VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrophoretic force on the particles exactly balances the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. Upon application of VB, the ink meniscus moves forwards to cover more of the ejection upstand 2. To eject the concentrated particles, a further voltage pulse of amplitude VP is applied to the ejection upstand 2, such that the potential difference between the ejection upstand 2 and the intermediate electrode 10 is VB+VP. Ejection will continue for the duration of the voltage pulse. Typical values for these biases are VIE=500 volts, VB=1000 V and VP=300 volts.
The voltages actually applied in use may be derived from the bit values of the individual pixels of a bit-mapped image to be printed. The bit-mapped image is created or processed using conventional design graphics software such as Adobe Photoshop and saved to memory from where the data can be output by a number of methods (parallel port, USB port, purpose-made data transfer hardware) to the printhead drive electronics, where the voltage pulses which are applied to the ejection electrodes of the printhead are generated.
One of the advantages of electrostatic printers of this type is that greyscale printing can be achieved by modulating either the duration or the amplitude of the voltage pulse. The voltage pulses may be generated such that the amplitude of individual pulses are derived from the bitmap data, or such that the pulse duration is derived from the bitmap data, or using a combination of both techniques.
Printheads comprising any number of ejectors can be constructed by fabricating numerous cells 5 of the type shown in FIGS. 1 to 3 side-by-side along the x-axis, but in order to prevent gaps in the printed image resulting from spacing between the individual printheads, it may be necessary to ‘overlap’ the edges of adjacent printheads, by staggering the position of the printheads in the y-axis direction. A controlling computer converts image data (bit-mapped pixel values) stored in its memory into voltage waveforms (commonly digital square pulses) that are supplied to each ejector individually. By moving the printheads relative to the substrate in a controllable manner, large area images can be printed onto the substrate in multiple ‘swathes’. It is also known to use multiple passes of one or more printheads to build up images wider than the printhead and to ‘scan’ or index a single printhead across the substrate in multiple passes.
However, stitch lines frequently result from the use of overlapped printheads or from overlapping on multiple passes and therefore it is known to use interleaving techniques (printing alternate single or groups of pixels from adjacent printheads or from different passes of the same or a different printhead) to distribute and hide the edge effects of the print swathes resulting from the overlapping ends of the printheads. It is generally recognised that a stitching strategy is necessary to obtain good print quality across a join between printed swathes. The known techniques rely on the use of a binary interleaving strategy i.e. a given pixel is printed by one printhead or the other. For example, alternate pixels along the x-axis are printed from adjacent overlapping printheads. Alternatively, a gradual blend from one swathe to the next can be used, by gradually decreasing the numbers of adjacent pixels printed from one printhead while increasing the numbers of adjacent pixels printed from the other printhead. This latter technique can be expanded by dithering the print in the y-axis direction. Another known technique is the use of a saw tooth or sinusoidal ‘stitch’ to disrupt any visible stitch line.
These techniques all represent different ways in which printing can be alternated between the nozzles of two overlapping printheads and the success of them depends on the droplet placement accuracy and registration of the two printheads, and is particularly sensitive to factors like substrate wander between lines of printheads. This can be mitigated by the dispersion and deliberate movement of the stitch to break up visible lines and disperse the errors over the width of the overlapping regions of the adjacent printed swathes.
An overlapping region between two swathes of print may be concealed by printing each pixel in the overlapping region with a contribution of ink from both printheads or passes, the two contributions adding to give the desired optical density for the specified greylevel of the respective image pixel. However, the optical density that results from the overlaying of two dots may not be equal to the optical density that results from one dot equal to the combined area of the two. Typically, a greater total volume of ink will be required for two overlaid dots to produce the same optical density as one dot. This causes problems for printing technologies that can only eject a limited number of droplet sizes or which form a printed dot from a discrete number of fixed-size droplets that combine on or before reaching the substrate to form a printed dot. Such methods have insufficient resolution of ejected volume to compensate for the change of optical density for pixels that are printed dot-on-dot in the overlapping region and would need to invoke a dithering regime between nearest available drop sizes to achieve the required optical density averaged over an area of many pixels, thereby compromising image resolution in the overlap region.